*{
    padding: 0;
    margin: 0;
    box-sizing: border-box;
}

div{
    margin: 0 auto;
    border-radius: 50%;
    padding-top: 50px;
}
.red{
    width: 400px;
    height: 400px;
    background: red;
}
.yellow{
    width: 300px;
    height: 300px;
    background: yellow;
}
.blue{
    width: 200px;
    height: 200px;
    background: blue;
}
.green{
    width: 100px;
    height: 100px;
    background: green;
}